Transcript E m = E k + E p

OMIT!
CHAPTER 3: DIFFERENT FORMS OF ENERGY
(1) WHAT IS ENERGY?
-The ability to do work or cause some type of change
-Unit: joules (J)
-1 J = 1 N x 1 m or 1 J = 1 Nm; where N: newton, m: metre
- 1 joule is a unit of energy equal to the work done when a force of one
newton acts through a distance of one meter. In other words, it's the amount
of energy exerted when a force of one newton is applied over a
displacement of one meter.
- Different forms of energy:
Form of Energy
Key word(s)
Example
Elastic
Stretched slingshot elastic
Electrical
Ener. stored due to
compression/extension
Electrons
Thermal
Random motion of particles
Toaster coils
Radiation
Electromagnetic waves
CT scan
Chemical
Molecular bonds
Plate of yummy spaghetti
Wind
Air movement
Wind
Sound
Sound waves
Larynx
Hydraulic
Water movement
Dam
Nuclear
Atomic nuclei
Plutonium
Battery
1.1 THE LAW OF CONSERVATION OF ENERGY
-Energy, ex. thermal, can 'travel' from one place to another: energy transfer
-A type of energy can change into another type of energy: energy transformation
-The total amount of energy in a closed system is always the same, thus:
Energy can never be destroyed or created, only transferred or transformed;
the total amount of energy is always constant
1.2 ENERGY EFFICIENCY
-Energy efficiency corresponds to the percent of energy that is actually used for
an intended purpose
-A car has an energy efficiency of about 12%, since the bulk of the energy is
used by parts and systems other than the wheels which actually make the car
move forward. In other words, about 88% of the chemical energy contained in
the fuel is lost to parts & systems such as the brakes, engine friction...
- A bicycle, on the other hand, is much more efficient since of the total energy
put in by the cyclist's muscles, about 90% of it goes directly to making the
wheels turn. In other words, there is only a 10% energy loss due to friction,
brake usage, turning handle bars...
Energy efficiency =
efficiency
x 100
Energy
1.3 THERMAL ENERGY
-The thermal energy in a substance depends on its temperature & on the amount
of particles it contains
-Thermal energy will be higher if:
(1) There are a higher number of particles involved.
(2) The substance's temperature is higher.
- Heat is the transfer of thermal energy from a warm environment to a cooler
environment, for example, when you are hugging a person who is cold & shivering,
you are giving heat to this shivering person who will warm up while you, on the
other hand, might feel the effects of this thermal energy transfer by feeling cooler
(The people on the right will conserve heat better since they are able slow down
the amount of heat lost to the cold water while the individual on the left will lose his
body heat faster to the water thereby reducing the amount of time he can last in
the cold water before getting rescued).
Equation for the relationship between thermal energy & heat:
Q = Et where Q is heat in joules (J), Et is the change in thermal energy in joules (J),
Greek letter delta means variation, Et thermal energy
THE DISTINCTION BETWEEN HEAT & TEMPERATURE
-Heat Temperature
-The difference is that:
Heat is determined by how fast particles are moving & the no. of particles present,
(joules)
Temperature takes only into account the speed of the particles, (degrees or
Kelvins)
THE RELATIONSHIP BETWEEN HEAT, MASS, SPECIFIC HEAT CAPACITY, &
TEMPERATURE VARIATIONS
-Consider 2 cubes of the same size made of different materials, for example iron
and silver. Even if both are exposed to the same degree of warmth they will not
warm up at the rate because iron & silver have different capacities to retain heat
or to let heat pass through it
-The amount of energy need to raise 1 gram of substance 1°C is its specific heat
capacity
-It therefore makes sense that metals have very low heat capacities because they
conduct heat very well and also because they will readily radiate to the
environment the heat they contain
-Table 3.6, p.75
- For example, it takes 4.19 joules of energy to raise 1 gram of water 1°C while it
takes only 2.46 joules to raise the same amount of ethanol (ethyl alcohol) 1°C.
The image at right shows this since if 1 kg of water and 1 kg of ethanol are
subjected to the same heat (energy), ethanol will become hotter since it is more
easily affected by temperature change
-Water's high heat capacity is crucial to our global climate (as you know) since
water will absorb large temperature changes without actually changing
temperature itself very readily
-The amount of heat released or absorbed by a substance can be calculated with
the following equation:
Q = mcT
where Q is heat in joules (J), m is mass in grams (g), c is the specific heat
capacity of a substance (J/g°C), T = Tf – Ti which is the change in temperature &
where Tf is final temperature and Ti is initial temperature, all 3 are in °C
-If Q is negative the substance has lost thermal energy; it has liberated heat;
it has less energy than it previously had
- If Q is positive the substance has gained thermal energy; it has absorbed
or taken in heat; it has more energy than it previously had
1.4 KINETIC ENERGY
-The ability to do work through MOTION
- Ek depends on mass & speed
Ek = ½ mv2 where Ek is in joules (J), m is mass in kilograms (kg) and v is velocity
(m/s)
- If mass of the object doubles, Ek doubles & if velocity of the object doubles, Ek
quadruples
1 J = 1 kg · m2/s2
1J=1N·m
1.5 POTENTIAL ENERGY
-The ability to do work due to an energy reserve
which comes from another type of energy
-HEIGHT is an important component; the higher the
height of the object is the greater its potential energy
will be
-Ep is often transferred into Ek in order to do the work
(right)
- Gravitational Ep (like in the problems we do)
incorporates an object's mass & height
Ep = mgh where Ep is in joules (J), m is mass in
kilograms (kg), g is gravitational field intensity which
is equal to 9.8 m/s2 for objects on Earth (see p.82 for
g value corresponding on other celestial bodies), h is
height in meters (m)
- If mass or height is doubled, Ep is doubled
1.6 MECHANICAL ENERGY
-The sum of the Ek & Ep of a system; Em always
stays constant throughout a system
Em = Ek + Ep
-As an object falls Ep decreases as height is lost,
at the same time Ek increases as velocity of the
object increases
-At the top of a cliff Ep = Em, since Ek is equal to 0
(no velocity since the object is not in motion); 1st
ball image at top
-Right before hitting the ground Ek = Em since
height is considered to be 0; last ball image
- Exactly midway in an object's fall Em = ½ Ek +
½ Ep; 3rd ball image in picture of previous page
- At each of the 3 positions below the mechanical energy is always constant
- Solve the following problem.
3.1 PRESSURE IN FLUIDS
- Pressure is the amount of force applied at right angle to the surface of an
object
P = F/A where P is pressure (Pa: pascals), F is force (N) & A is the area
experiencing the force (m)
- 1 Pa = 1N/1m2 or 1 pascal is the force applied to an area of 1m2
PRESSURE IN A LIQUID
-Pressure depends on:
1. Depth: the deeper an object is in a liquid the higher the P will be on that
object; the shallower an object is the less the P will be
2. Density: the higher the density of the liquid is, the greater the P will be on a
submerged object; the lower the density of the liquid is, the less P a submerged
object will experience
- Because of the higher density of sea water (as opposed to fresh water),
oceanic drivers experience higher pressure during dives
PRESSURE IN A GAS
-P depends on the no. of particle collisions between gas particles; the more
collisions there are, the more P there will be
-Particle collisions are dependent on:
1. Temperature: the higher the T of a gas; the more collisions there will be &
vice versa
2. Volume: the greater the volume of a gas is, the lower the P will be & vice
versa
3.2 PASCAL'S PRINCIPLE
-P applied to an enclosed liquid is transmitted equally in the liquid & equally to
the walls of the container
-Ex. a hydraulic lift, image right, used in a garage to do maintenance or repairs
on a car
-This phenomenon occurs because liquids are fluids which are difficult to
compress
- Here a small piston is pushed on a certain distance; the P generated is
transferred to a larger piston which move a greater load but over a much
smaller distance
3.3 ARCHIMEDE'S PRINCIPLE
-An object dropped in a liquid can: sink or float
-Archimedes discovered that:
a) The volume of an object can be determined by measuring the water the object
displaces (overflow method);
b) P in a liquid increases with depth creating an upward force called buoyancy;
c) The P created by the water on top of a submerged object is lower than the P
found below a submerged object since at this lower point depth is greater, thus P is
greater creating a resultant force that goes upwards
d) Archimedes' Principle:
Buoyant force = weight of the fluid displaced by a submerged object
- 3 situations possible:
1. Fb (buoyant force) < Fg (force of gravity = object's mass x gravitational factor of 9.8 m/s2);
resultant force points downwards – object sinks
2. Fb = Fg; net (resultant) force = 0; object stays at same place in water
3. Fb > Fg; resultant force points upwards – object rises to the surface
- The shape of an object will have an influence on its floatability; a heavy boat
can float since the large surface area coming in contact with the water (the
bottom of the boat) increases the magnitude of the buoyancy force; a block of the
same weight as the boat will sink since the buoyant force is applied to a smaller
surface area therefore the object's weight cannot be lifted by the buoyancy force
(see similar example below, boat will float & the ball will sink)
- Another example of Archimedes' principle at work is the ballast tanks in a
submarine enabling a sub to sink or float at will; when tanks fill with water the
sub sinks; when tanks are filled with air, the sub's density decreases & it rises in
the water
3.4 BERNOUILLI'S PRINCIPLE
- P of a moving fluid varies with its speed; an increase in speed means less P
-Air moving over a curved surface will have a higher velocity (top of airplane
wings in images below); meaning the P on top of the wing will be lower; air
moving below the wings is moving across a straight surface, the air particles
will move slower creating higher P
-The resultant force is one of LIFT since the high P force below the wing will
push upwards enabling the plane to stay in the air
- This occurs because the air particles themselves do not move much; rather
it is the plane's wing which slices through the air. As the top curved surface
hits the air particles they are given an extra push and will move faster creating
an area of low P